Starting Fuel Enrichment Valve
RPM, If both elements are operating properly,
the paralleling light will be on between 16 percent
The primer switch (with two positions, ON
and 65 percent RPM only. If the secondary
and spring-loaded OFF) operates the fuel
element fails, the light never comes on; if the
enrichment valve, providing increased fuel flow
primary element fails, the light is on above 65
during engine starting. The primer switch must
percent RPM.
be in the ON position and held there before the
engine reaches 16 percent RPM. Further, it must
Fuel Control
remain on until the fuel control shutoff valve
opens at 16 percent RPM or enrichment will not
The fuel control is on the accessories drive
occur. The enrichment (primer) valve closes when
housing and mechanically links to the coordi-
fuel pressure in the fuel manifold reaches 50 PSI.
nator. The fuel control provides a starting fuel
Fuel enrichment is needed only in very cold
flow schedule that, in conjunction with the
climates.
temperature datum valve, prevents overtempera-
ture and compressor surge.
The fuel control schedule is 20 percent richer
Temperature Datum Valve
than the nominal engine requirements to accom-
modate the temperature datum valve. The valve
The temperature datum valve is located
bypasses 20 percent of the control output when
between the fuel control and the fuel nozzles. It
in the null position. This excess flow gives the
is a motor-operated bypass valve that responds
temperature datum valve the capacity to add as
to signals from the temperature datum control.
well as subtract fuel. The valve is then able to
If the power lever positions are between 0 degrees
maintain the temperature scheduled by the
and 66 degrees, the valve remains in null and the
coordinator and the temperature datum control.
engine operates on the fuel flow scheduled by the
The fuel control includes a cutoff valve for
fuel control. The valve remains in null unless the
stopping fuel flow to the engine. It actuates either
temperature datum control signals it to limit
manually or electrically. During engine starts, the
turbine inlet temperature. The valve then reduces
cutoff valve remains closed until the engine
the fuel flow (up to 50 percent during starting,
reaches 16 percent RPM. The speed-sensitive
20 percent above 94 percent RPM) by returning
control then opens the cutoff valve, permitting
the excess to the fuel pump. When turbine inlet
fuel to flow to the engine.
temperature is at the desired level, the temperature
datum control signals the valve to return to the
null position.
Fuel Nozzles
In power lever positions between 66 degrees
The fuel output from the temperature datum
and 90 degrees, the temperature datum valve acts
valve flows through the fuel manifold to the six
to control turbine inlet temperature to a
fuel nozzles. Fuel flows through both the primary
preselected schedule corresponding to power lever
and secondary nozzle orifices during normal
position. This is the temperature controlling
operation. At low fuel flow rates, fuel flows
range. In this range the temperature datum
through the primary orifice only.
control may signal the valve to allow more (higher
temperature desired) or allow less (lower
temperature desired) fuel to flow.
Fuel Control Relay
The fuel control relay is a fail-safe-type relay
Drain Valves
that energizes when the FUEL and IGNITION
switch is off, or when the propeller is feathered.
A spring-loaded, solenoid-operated manifold
With this arrangement, the engine can still operate
drain valve is located at the bottom of the fuel
with an electrical power failure during flight. The
manifold. It drains the fuel manifold when fuel
pilot shuts the engine down by placing the FUEL
pressure drops below 8 to 10 PSI. This action
and IGNITION switch in the OFF position, or
minimizes the amount of fuel dropping into the
by feathering the propeller. Power then goes to
combustion liners while the engine unit is being
the fuel control shutoff valve, which closes and
stopped.
stops all fuel to the engine.
